The present invention relates to crane assemblies and, more particularly, relates to crane assemblies which are mounted on trucks or other movable platforms and which have a vertically pivotable boom mounted on a rotatable turntable and a cable hoist assembly mounted on the boom.
So-called boom truck cranes are well known. Referring to FIG. 8, a conventional assembly typically includes a pedestal 220 mounted on or proximate the bed 202 of a truck 204. A turntable assembly 230 is mounted on the pedestal 220, and a boom 280 and a hydraulic piston and cylinder device 300 are mounted on the turntable assembly 230. A hoist 250 is mounted on the lower or front end of the boom 280 and is actuatable to raise and lower a hook block or down haul weight 320 by winding and unwinding a cable from a drum provided on the hoist 250. An A-frame cradle or other similar boom rest 330 is provided on the rear end of the truck bed 202 and receives the boom 280 when the boom is lowered to the horizontal position, thus storing the boom for transport of the crane assembly.
The turntable assemblies, hoist mount assemblies, and boom rests of conventional truck mounted boom assemblies, though satisfactory, exhibit several drawbacks and disadvantages.
First, turntables of such assemblies must be rotatably driven by drive systems including motors, gear trains, brakes, etc. In order to prevent externally induced rotation which could otherwise occur during transport of the vehicles and which could overload such drive systems and damage this machinery, the turntable assemblies must be mechanically locked from rotation. The typical boom assembly of the type illustrated in FIG. 8 provides this locking by providing a cradle-type boom rest 330 of the type described above. Rotation of the turntable is prevented during vehicle transport by supporting much of the weight of the boom 280 on the boom rest 330, thus providing sufficient resistance to torque to prevent unintended rotation of the turntable assembly 230 with respect to the pedestal 220. While this arrangement is functionally adequate to prevent rotation, the provision of the cradle 330 on the truck bed 202 significantly reduces the utility of the truck bed for cargo carrying purposes. In addition, actuation of the piston and cylinder device 300 and lifting of the boom 280 from the cradle 330 is required to allow for rotation of the turntable assembly 230.
Other devices have been proposed to lock a turntable assembly of a truck mounted crane assembly from rotation without resting the boom on a cradle or other support located remote from the turntable. One such device is disclosed in U.S. Pat. No. 3,648,956, which issued to Loree A. D. Paulson on Mar. 14, 1972. Pursuant to this design, a pin is provided which extends through the side of the turntable and which is insertable into an aperture located in the boom support assembly. A handle is attached to the end of the pin which protrudes from the turntable. When the handle is manually moved away from the boom support assembly, the pin is withdrawn from the support assembly and thus allows rotation of the turntable with respect to the support assembly.
While this arrangement has the advantage of locking a turntable without requiring a cradle assembly for supporting the boom, it still exhibits several disadvantages. Most notably, the handle and pin must be manually withdrawn, without the aid of any other mechanical device, by pulling the handle away from the turntable support. This requires substantial manual effort and may inhibit unlocking of the pin if substantial residual forces are imposed on the pin by the boom mount assembly. Moreover, no means are provided for maintaining the handle and pin in their withdrawn positions, thus requiring the operator to hold the handle in its retracted position while the turntable is rotated.
As discussed above, the cradle assembly associated with conventional truck mounted boom assemblies has the disadvantage of limiting the cargo carrying capacity of the truck on which the crane is mounted. However, even if some other device were to be provided to lock the turntable to the pedestal of such an assembly, the cradle still could not be removed since the boom itself still would have to be stowed for transport. More specifically, referring again to FIG. 8, a crane boom 280 must be supported during transport to prevent damaging pressures from being induced in the hydraulic lift cylinder of the piston and cylinder device 300 caused by internal forces of the boom as the truck 204 travels over various terrain. To avoid these damaging forces, at least some of the weight of the boom must be taken off from the piston of the piston and cylinder device 300. This is accomplished in the conventional device by supporting the free end of the boom 280 on the cradle type device 330 discussed above. Thus, the cradle device cannot be removed in conventional assemblies without risking damage to the piston and cylinder device used to raise and lower the boom.
Proposals have been made to eliminate the conventional cradle-type boom rest assembly by providing a structure which supports the boom for transport and which is not directly supported on the bed of the truck. For instance, in U.S. Pat. No. 4,579,235, which issued to Herbert L. Orwig on Apr. 1, 1986, a boom rest device has been proposed which comprises a pair of arms which extend generally in the same direction as the piston and cylinder device, which are connected at first ends thereof to the boom, and which have second ends which are connectable to the pedestal or pedestal mount assembly to support the boom for transport.
While the boom rest disclosed by Orwig does eliminate the need for the provision of a cradle on a truck bed, it still exhibits several disadvantages. Most notably, for connection of the arms to the base member, the arms must be lowered to a predetermined position and maintained in that position while the boom is lowered to a position in which the arms engage mounting bosses on the pedestal. This positioning is accomplished in Orwig by the provision of chains which hold the arms in the proper orientation for lowering. However, these chains are subject to breakage or kinks which could prevent adequate mating of the arms to the bosses which they engage. This problem is compounded by the fact that the two arms act independently of one another, thus doubling the chances of complications.
Another disadvantage associated with the conventional truck mounted boom assembly is the manner by which the hoist is mounted on the end of the boom. Referring again to FIG. 8, the typical hoist 250 is bolted directly to the end of the boom 280 via fixed mounting flanges. To expose the internal workings of the rear end of the boom 280 for adjustment, inspection, or maintenance, the hoist 250 must be unbolted from the mounting flanges and removed. Since the hoist weighs several hundred pounds, an external lifting apparatus such as another crane must be employed to remove the hoist and to lift it back into position for reattachment. Even then, considerable manual effort is required to properly position the hoist for reattachment to the boom.